Dark Sectors at the Fermilab SeaQuest Experiment

TL;DR

The paper demonstrates that the SeaQuest fixed-target experiment, with modest upgrades (ECAL), can probe GeV-scale dark sector scenarios including dark photons, dark Higgs, iDM, leptophilic scalars, and ALPs through displaced decays behind shielding. They model production channels (meson decays, Bremsstrahlung, Drell-Yan), and compute geometric acceptance with KMAG effects, showing Phase I/II reach comparable to NA62, SHiP, FASER. They provide detailed estimates of backgrounds, fiducial regions, and signal yields, highlighting that an ECAL-laden enhanced SeaQuest can achieve largely background-free searches for displaced electrons and other final states. The results underscore SeaQuest's potential as a low-cost, rapid-testbed for light dark sectors and encourage dedicated detector studies to optimize the experimental layout and signal reach.

Abstract

We analyze the unique capability of the existing SeaQuest experiment at Fermilab to discover well-motivated dark sector physics by measuring displaced electron, photon, and hadron decay signals behind a compact shield. A planned installation of a refurbished electromagnetic calorimeter could provide powerful new sensitivity to GeV-scale vectors, dark Higgs bosons, scalars, axions, and inelastic and strongly interacting dark matter models. This sensitivity is both comparable and complementary to NA62, SHiP, and FASER. SeaQuest's ability to collect data now and over the next few years provides an especially exciting opportunity.

Figures (15)

• Figure 1: Layout of the SeaQuest spectrometer in its current form (adapted from Ref. Gardner:2015wea).
• Figure 2: Number of dark photons (solid color) produced at Phase I of SeaQuest ($1.44\times 10^{18}$ POT) in various production channels for $\epsilon=10^{-6}$. An estimate of the theory uncertainty for proton Bremsstrahlung is shown as the shaded blue region (see text for details). For comparison, we also show the analogous production rate for electron Bremsstrahlung (dashed gray), assuming a 120 GeV electron beam, $1.44 \times 10^{18} \text{ EOT}$, and production within the first radiation length of a tungsten target.
• Figure 3: In minimal dark photon models, the geometric efficiency for dark photons produced from proton Bremsstrahlung (blue) and the decays of pions (red) or eta mesons (orange). For each of these production channels, we assume that the $A^\prime$ decays to an electron pair after traveling $5.5 \text{ m}$ (bottom line), $10.5 \text{ m}$ (middle line), or $12 \text{ m}$ (top line). For these latter two decay points, the electron pair only traverses a fraction of the KMAG magnet.
• Figure 4: Signal kinematics of $A^\prime \to e^+ e^-$ for dark photons produced from exotic eta meson decays (orange) and proton Bremsstrahlung (blue). The left (right) panel displays energy (angular) distributions for electrons originating from dark photon decays before traveling through KMAG. The solid (dashed) line corresponds to $m_{A^\prime} = 0.01 \text{ GeV}$$(0.5 \text{ GeV})$. The vertical gray dotted line in the right panel denotes the angular scale of the SeaQuest spectrometer.
• Figure 5: Left panel: The projected Phase I SeaQuest sensitivity to the dark photon parameter space using the $5 \text{ m} - 6 \text{ m}$ fiducial decay region. The various contours correspond to 10 dielectron signal events for dark photons produced from meson ($\pi^0,\eta,\eta^\prime, \omega$) decays and proton Bremsstrahlung. The blue shaded region represents the theoretical uncertainty in computing the Bremsstrahlung rate (see text for details). Right panel: Seaquest sensitivity to displaced dark photons at Phase I (solid purple) and Phase II (dashed purple), corresponding to 10 signal events. For Phase I, we conservatively fix the fiducial decay region to $5 \text{ m} - 6 \text{ m}$. For Phase II, moving from darker to lighter contours corresponds to the fiducial decay regions of $5 \text{ m} - 6 \text{ m}$, $5 \text{ m} - 9 \text{ m}$, and $5 \text{ m} - 12 \text{ m}$, respectively. The gray region denotes parameter space that is already excluded by past experiments Battaglieri:2017aumAlexander:2016aln.